1. Undecideds will break for Murphy, who is not the candidate whose disapproval rating is over 40%.
2. Hispanics will end up voting for Murphy instead of Rubio by a higher margin, largely because of Trump.
3. Trump will implode even more because of new scandals, dragging Rubio down with him. See also: Joe Heck in Nevada.
Rubio is probably up by something like 4 points right now, which is nothing a new Trump scandal can't "solve". The main reason Dems pulled out of this race is because there are so many tossups where it's cheaper to invest than in Florida, and Rubio should be grateful for that. This race is a tossup and I fully expect it to go to Murphy in the end.
I don't think Rubio is quite controversial or unpopular enough for undecideds to break heavily
enough for Murphy, though. If polls of Latinos are to be believed, Rubio isn't only winning among Latinos overall, he's tied with Murphy among Puerto Ricans, who are normally quite Democratic-leaning.
As for Heck tanking, we're already seeing polling evidence of that, and Heck unendorsed Trump. He's probably slipping because some Trump voters are refusing to support him. Rubio doesn't have to worry about that. Perhaps if Trump loses FL by at least 7-8%, Rubio will be in trouble, but it hard to see a state as large as FL swinging Democratic by that much.
I'm resigned to reality at this point, which is that Rubio is overwhelmingly favored, and hoping for Murphy to win would just be setting myself up for disappointment. This late in the game, I'd rate this Likely R.